Introduction

- Tissue engineering of cartilage in space

Exposure of humans to microgravity affects cells and tissues at a variety of levels (1, 2). Musculoskeletal changes (e.g., significant bone and muscle loss) occur even when astronauts exercise regularly, but the mechanisms are not yet understood (3, 4). Tissue engineering, a new field that enables tissue equivalents to be created from isolated cells in combination with biomaterials (5, 6) and bioreactor culture vessels (7), potentially can provide a basis for systematic, controlled in vitro studies of tissue growth and function. We developed a method to create three-dimensional cartilaginous tissue based on cartilage cells (chondrocytes), biodegradable polyglycolic acid (PGA) scaffolds (8), and bioreactors (7, 9). The scaffold induces cell differentiation and degrades at a defined rate, whereas the bioreactor maintains controlled in vitro culture conditions that permit tissue growth and development.

In the present work, we studied tissue engineering in space by using cartilage as a model musculoskeletal tissue. Cartilage was selected because of its resilience and low metabolic requirements (10). Previous microgravity experiments involving mammalian cells focused on the cells themselves in monolayers and lasted for 6-28 days (11). In the present study, three-dimensional engineered cartilage grown for 3 months on Earth followed by 4 months on the Mir Space Station at 104-106g (11) was compared with that grown for 7 months on Earth at 1 g (Fig. 1). The objectives were to: (i) examine chondrocyte viability and differentiated function in a long-term flight study, and (ii) assess the effects of the space environment which, in the present experiment and during human spaceflight, includes exposure to microgravity as well as launch and landing, on cartilage growth and function.